Regarding the spelling
EID. (=
EIDIBUS, not
EDIBUS as the post says), I think there is some evidence that spelling was more conservative in the late Republican period than our manuscripts and editions would indicate (e.g. Quintilian's statement about the use of geminate
s in Cicero's time (
caussa,
cassus,
divissiones) which is completely unattested in our manuscripts). Even beyond that, inscriptions and coins tended to conserve archaic spellings more than other texts, and
EI for long
I is pretty common in inscriptions.
Here's an interesting coin from the Augustan age commemorating an old treaty with Gabii (
foedus p.r. qum Gabinis), where "cum" is spelled "qum". Though, as a professor of a friend of mine pointed out, this is less likely to indicate a genuine spelling and more a deliberate attempt at archaism, like saying "ye olde treatie" in English.